Abstract: A propping agent for fractures produced in the walls of a bored well comprising a granular or particulate product containing on the basis of its oxide content up to 85% by weight zirconium oxide (ZrO.sub.2) in a quantity which is such that the weight ratio of ZrO.sub.2 /SiO.sub.2 is equal to or greater than 1.5, alumina (Al.sub.2 O.sub.3) in a quantity which is such that the weight ratio Al.sub.2 O.sub.3 /SiO.sub.2 is between 0 and 1.5 and sodium oxide (Na.sub.2 O) in a quantity which is such that the weight ratio Na.sub.2 O/SiO.sub.2 is between 0 and 0.04.